Re: [Xen-devel] [PATCH] mm/x86: Flush lazy MMU when DEBUG_PAGEALLOC is set

[Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>]

On Wed, Feb 27, 2013 at 02:40:01PM -0800, H. Peter Anvin wrote:
> Greg, policy opinion?
> 
>       -hpa
> 
> On 02/26/2013 03:57 PM, Boris Ostrovsky wrote:
> > 
> > ----- hpa@xxxxxxxxx wrote:
> > 
> >> On 02/26/2013 02:56 PM, Boris Ostrovsky wrote:
> >>> When CONFIG_DEBUG_PAGEALLOC is set page table updates made by
> >>> kernel_map_pages() are not made visible (via TLB flush) immediately
> >> if lazy
> >>> MMU is on. In environments that support lazy MMU (e.g. Xen) this may
> >> lead to
> >>> fatal page faults, for example, when zap_pte_range() needs to
> >> allocate pages
> >>> in __tlb_remove_page() -> tlb_next_batch().
> >>>
> >>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> >>> ---
> >>>  arch/x86/mm/pageattr.c | 2 ++
> >>>  1 file changed, 2 insertions(+)
> >>>
> >>> diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
> >>> index ca1f1c2..7b3216e 100644
> >>> --- a/arch/x86/mm/pageattr.c
> >>> +++ b/arch/x86/mm/pageattr.c
> >>> @@ -1369,6 +1369,8 @@ void kernel_map_pages(struct page *page, int
> >> numpages, int enable)
> >>>    * but that can deadlock->flush only current cpu:
> >>>    */
> >>>   __flush_tlb_all();
> >>> +
> >>> + arch_flush_lazy_mmu_mode();
> >>>  }
> >>>  
> >>>  #ifdef CONFIG_HIBERNATION
> >>>
> >>
> >> This sounds like a critical fix, i.e. a -stable candidate.  Am I
> >> correct?
> > 
> > I considered copying stable but then I decided that this is a debugging 
> > feature
> > --- kernel_map_pages() is only defined if CONFIG_DEBUG_PAGEALLOC is set and 
> > my
> > thinking was that stable kernels usually don't do this.

"Stable" kernels are used all over the place, like in distros, which
might enable this.

I have no objection to taking this patch in a stable release, as it does
fix a real problem.

thanks,

greg k-h

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